1. Field of the Invention
This invention relates to a process for the preparation of crystalline calcium sodium metaphosphate fiber. More particularly, this invention relates to a process for the preparation of asbestiform crystalline calcium sodium metaphosphate fiber. Such fiber is characterized by having the form or appearance of asbestos and by exhibiting an average aspect ratio of at least 10, preferably at least 50, and an average diameter from about 0.5 micron (.mu.m) to about 20 .mu.m.
Asbestiform crystalline calcium sodium metaphosphate fiber is suitable for use as a replacement fiber in many applications which historically have employed asbestos and other durable inorganic fibers as inorganic fibrous insulation and reinforcement materials.
2. Description of the Prior Art
Numerous durable inorganic fibers--asbestos, glass fibers, for example--are used in a wide variety of applications. Common among such applications are composites with organic polymeric material such as plastics, resins, natural and synthetic elastomers, and the like. Such composites are used to make floor tiles, gaskets, brake linings, clutch facings, and numerous other objects commonly found in industry and in the home.
The use of durable inorganic fibers such as asbestos and glass fibers as the inorganic fibrous material in the aforementioned applications in general provide satisfactory performance. However, such fibrous materials also suffer from certain disadvantages. For example, many of such fibrous materials, especially asbestos, have been found to expose workers as well as the general public to a potentially serious health hazard. It has been determined that the inhalation of small asbestos fibers can result in a disease known as asbestosis in which these fibers accumulate in the lungs, scar lung tissue, and cause many respiratory problems. It has become increasingly clear that inhalation of asbestos fibers over an extended period of time can lead to a cancer of the lining of the lungs known as mesothelioma as well as lung cancer. Numerous other durable inorganic fibers, for example, alkali metal titanates, are also suspected of causing similar health problems among users of such fibers. In addition, many inorganic fibers are rapidly soluble in water, dilute acids or dilute alkali which limits the usefulness of such fibers.
Accordingly, there exists a great need for an inorganic fibrous material suitable for use as an insulation fiber and as a reinforcing fiber in applications requiring the presence of inorganic fibrous materials which are not rapidly soluble in water, dilute acid or dilute alkali, and which do not present an undue health hazard to those exposed to such materials. Asbestiform crystalline calcium sodium metaphosphate fiber satisfies each of these requirements.
A unique feature of asbestiform crystalline calcium sodium metaphosphate fiber is the presence of a polyphosphate backbone which is believed to account for the lack of an undue health hazard to those exposed to such fiber. Polyphosphate chains, of course, are known to be biodegradable. As such, it is believed that, in contrast to durable inorganic fibers, biodegradation by enzyme-assisted hydrolysis of the polyphosphate backbone of asbestiform crystalline calcium sodium metaphosphate fiber should occur in vivo to facilitate dissolution thereof. The resulting calcium, sodium, and inorganic phosphate can then be utilized in the normal biochemical functions of the cell.
A process to prepare asbestiform crystalline calcium sodium metaphosphate fiber is known to the art. In U.S. Pat. 4,346,028, such fiber and a process to prepare same are disclosed. The process comprises forming a melt of a source of oxygen, calcium, phosphorus, and sodium having a mol percent ratio of about 15 mol percent to about 30 mol percent sodium oxide (Na.sub.2 O), about 48 mol percent to about 60 mol percent phosphorus pentoxide (P.sub.2 O.sub.5), and about 20 mol percent to about 37 mol percent calcium oxide (CaO), maintaining the resulting mixture at a temperature below the melting point of the calcium sodium metaphosphate for a time sufficient to form blocks of calcium sodium metaphosphate, and fiberizing the blocks into asbestiform crystalline calcium sodium metaphosphate fiber.
Although asbestiform crystalline calcium sodium metaphosphate fiber prepared by the process of the prior art is effective for contemplated end use applications, a process which can be readily adapted for batch, semicontinuous, or continuous operations would provide for increased efficiency in any commercial scale operation. Accordingly, research efforts are continually being made to define new or improved processes to more efficiently prepare asbestiform crystalline calcium sodium metaphosphate fiber. The discovery of the process of the instant invention provides such a process and, therefore, is believed to be a decided advance in the asbestiform crystalline calcium sodium metaphosphate fiber art.